I have returned several rooted phones to verizon with no problems. I first unroot, then in system do a system reset. if you do install a rom be sure to flash back to the stock android that came with it.

My droid x for instance, I run roms etc, I flash back stock, then reset system, ok no problems with verizon

On the dx2 I have returned my gf's phone that I rooted for her. I just unrooted and reset system and okEdited by 66racer - 2/8/12 at 7:44pm

With HTC, certain phones can only be unlocked using the HTC unlock method provided by them. My EVO 3D for example, I had to do this and the HTC method leaves a watermark that indicates a root. Even if I unroot it, the watermark is still there.

Most other methods will allow you to flash the OEM ROM and unroot the phone without a problem.